simple expression parser in C

parser.c

// Top-level declarations
static double parse_add(void);
static const char* state;

// Checks if state is a valid digit
static int is_digit(void) {
	return *state >= '0' && *state <= '9';
}

// Get the digit value of state
static int digit(void) {
	return *state - '0';
}

// Parses a number
static double number(void) {
	double result = 0;
	
	while (is_digit()) {
		int n = digit();
		
		result *= 10;
		result += n;
		
		++state;
	}
	
	if (*state == '.') {
		++state;
		double dec = 0.1;
		
		while (is_digit()) {
			int n = digit();
			
			result += n * dec;
			dec *= 0.1;
			
			++state;
		}
	}
	
	return result;
}

// Parses a factor (unary, parenthesis, and number)
static double parse_factor(void) {
	switch (*state) {
		case '+': ++state; return  parse_factor();
		case '-': ++state; return -parse_factor();
		case '(': {
			++state; // eat (
			double result = parse_add();
			++state; // eat )
			
			return result;
		}
		default: return number();
	}
}

// Parses * and /
static double parse_mul(void) {
	double left = parse_factor();
	
	while (*state == '*' || *state == '/') {
		char op = *state++;
		double right = parse_factor();
		
		if (op == '*')
			left *= right;
		else
			left /= right;
	}
	
	return left;
}

// Parses + and -
static double parse_add(void) {
	double left = parse_mul();
	
	while (*state == '+' || *state == '-') {
		char op = *state++;
		double right = parse_mul();
		
		if (op == '+')
			left += right;
		else
			left -= right;
	}
	
	return left;
}

// Eval function
static double eval(const char* string) {
	state = string;
	return parse_add();
}

// Simple reading from stdin. Only the stuff above is relevant.
#include <stdio.h>

static int eval_line(void) {
	char buffer[1024], c;
	int length = 0;

	printf("> ");

	while (c = getchar(), c != '\n') {
		buffer[length] = c;
		
		// Ignore whitespaces
		if (c != ' ')
			++length;
	}

	buffer[length] = 0;

	if (length > 0)
		printf("%f\n", eval(buffer));

	return length;
}

int main() {
	while (eval_line() > 0);
	return 0;
}